They have discovered evidence of two supermassive black holes at the centre of two nearby galaxies both of which are concealed behind clouds of gas and dust.

They were spotted by NASA's orbiting observatory NuSTAR, which is designed to see x-rays.

Data from NuSTAR - the Nuclear Spectroscopic Telescope Array - was then analysed by British researchers at the universities of Durham and Southampton.

Ady Annuar, an astronomer at the University of Durham's Centre for Extragalactic Astronomy, said: "These black holes are relatively close to the Milky Way, but they have remained hidden from us until now.

"They're like monsters hiding under your bed.

"Their recent discoveries certainly call out the question of how many other supermassive black holes we are still missing, even in our nearby universe."

Image:The nearest black hole was found 38 million light years away from the Milky Way

In black holes gravity is so powerful it traps light and distorts time and space.

They can only be detected from the last gasp emissions of radiation from objects which fall into them.

The first of the new black holes was discovered in galaxy NGC 1448, which is "just" 38 million light years away from our own galaxy, the Milky Way.

The other is in the galaxy IC 3639 which is 170 million light years away.

Both emit intense levels of radiation and are classified as active galaxies.

Scientists now believe that most large galaxies may have supermassive black holes at their core, but many of these are hidden from view.

NuSTAR project scientist Daniel Stern, said: "It is exciting to use the power of NuSTAR to get important, unique information on these beasts, even in our cosmic backyard where they can be studied in detail."